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The topic of unauthorized immigration is undoubtedly a controversial one. It is generally stated, but somewhat disputed, that around 11 million unauthorized immigrants currently reside in the United States.

However, even if the stock of unauthorized immigrants were estimated imprecisely, to the extent that the estimation is based on a consistent methodology, it would still be possible to use this information to accurately track changes in the number of unauthorized immigrants throughout the years.

The implementation and design of immigration policies critically depends on reliable information about the size and characteristics of the unauthorized population, including their region of residence, where they work, and what kind of job they do. From a public policy perspective, this information is extremely useful when deciding how to allocate resources between border and interior enforcement of immigration laws, for example.

From a research perspective, accurate information on unauthorized immigrants is key to quantitatively and qualitatively assessing the social and economic effects of unauthorized immigration on the native-born population and on the provision of local public services, such as education and health care. Moreover, this information is relevant to understand the factors that drive individuals to migrate unlawfully into the United States.

How are Unauthorized Immigrants Counted?

Counting the number of unauthorized immigrants is, of course, a very challenging endeavor. In the United States, unauthorized immigration typically takes place in three ways: (i) visitors or students who overstay on valid non-immigrant temporary visas or violate the term of admission; (ii) immigrants admitted based on fraudulent documentation; and (iii) those who enter the United States by crossing the borders and avoiding inspection from U.S. border officials.

Statistics on unauthorized immigration are published by several organizations. The Department of Homeland Security’s (DHS) Office of Immigration Statistics (OIS), Pew Research Center (Pew), and Migration Policy Institute (MPI) are the most frequently cited sources. Their methodologies vary slightly, but the estimates are quite similar. In all cases, the number of unauthorized immigrants is estimated using a residual method. According to this approach, the estimate is obtained as the difference between the total number of foreign-born population in the United States and the number of immigrants who entered the country and currently reside lawfully.

The year 1980 is typically assumed as the starting year of all estimates. The reason is that most of the people who entered illegally into the United States before 1980 were later eligible for and were granted legal permanent residence status, so they effectively became lawful immigrants. Data on the total number of foreign-born population present in the country, both authorized and unauthorized, are obtained either from the Current Population Survey or the American Community Survey. The information on the number of lawful entries is very reliable. Immigrants enter the country as permanent residents under two categories: green card holders or refugees. Information on green card holders comes from the DHS, and information on refugees comes from the Office of Refugee Resettlement.

The data used in the calculation of the number of unauthorized immigrants contain detailed demographic, labor market, and geographic information, so the estimates can be disaggregated by gender, age, productive sector, and location. The final numbers are generally adjusted to account for several issues, including the fact that the survey may miss some unauthorized immigrants.

The chart above, produced by Pew, shows estimates of the number of unauthorized immigrants for different years. According to their estimates, the number reached a maximum of 12.2 million in 2007 and later declined to stabilize between 11 million and 11.5 million. It is interesting to observe that this stabilization occurred concomitantly with a decline in the total number of apprehensions of unauthorized immigrants, both at the border and internally. Such trend is evident from the graph below.

The share of unauthorized immigrants as a proportion of the total population in the United States was 3.5% in 2014. Within the district, the shares were 3.9 percent in Washington, D.C. (ranked 10th in the country), 4.2 percent in Maryland (ranked 7th), 3.4 percent in North Carolina (ranked 18th), 1.8 percent in South Carolina (ranked 31st), 3.5 percent in Virginia (ranked 14th), and 0.2 percent in West Virginia (ranked 51st). The chart below shows the total number of unauthorized immigrants from 1990 to 2014 in District jurisdictions. (Note: Data for West Virginia was not available.)

Research on Unauthorized Immigration

The ongoing research on unauthorized immigration is quite large and covers different areas. More recently, there has been particular attention devoted to study and understand the impact of state policies on unauthorized immigration. For instance, some work attempts to quantify the effects on labor market outcomes for both domestic workers and unauthorized immigrants of implementing programs such as E-verify. Other work focuses on how state and federal policies - such as certain public goods and welfare benefits, and border and internal enforcement of immigration laws - affect the localization decisions of unauthorized immigrants.

One question that is relevant for the design and implementation of immigration enforcement policies is who should be in charge of enforcing immigration laws. This question is of great interest, especially in the context of the United States, where several states (Arizona, Alabama, South Carolina, and others) have promoted different initiatives granting state governments a more important role in controlling unauthorized immigration. A similar situation is observed in the European Union (EU). The EU has been developing common immigration rules since 1999, but individual EU nations still retain authority on several aspects of immigration policy (for example, concerning immigration from non-EU countries.)

So, what role should different levels of governments play in enforcing immigration laws and what are the implications of adopting different institutional arrangements in the allocation of responsibilities?

In a recent paper with Subhayu Bandyopadhyay of the St. Louis Fed, we examine the implications of shifting from centralized enforcement of immigration laws (i.e., enforcement by the federal government in the case of the United States, or by a supranational government in the case of the EU) to an institutional arrangement in which individual state governments (or individual countries for the EU) decide, in a decentralized way, on the levels of border and interior enforcement.

In the paper, we characterize the case of the United States by developing a framework of analysis where unauthorized immigrants first choose among alternative entry points or border states, and, once in the country, they choose a (potentially different) final destination within the United States. In other words, the state of entry may not coincide with the final residential location for immigrants. The paper shows, among other things, that when states have the responsibility of deciding the amount of resources to devote to the enforcement of immigration policies, they tend to choose relatively high levels of internal enforcement and relatively low levels of border enforcement compared to the levels chosen by the federal or central government.

Moreover, to the extent that unauthorized immigrants benefit from the provision of local public goods (such as local schools and health services) but do not entirely pay for the cost of their provision, state governments acting individually will decide to offer lower levels of local public goods compared to the centralized case. The rationale behind these results is that policies chosen by state governments generate spillover effects on other regions, since they induce a geographical relocation of unauthorized immigrants. When a state government decides its policies, it neglects the impact they may have on other states. Such "wasteful competition" between state governments disappears when a central government makes those decisions.

The paper also shows that the number of unauthorized immigrants in the country is higher in the decentralized case than in the centralized case, which, in turn, affects domestic workers and firms in conflicting ways. Particularly, wages tend to be lower and profits tend to be higher when enforcement is decided by each individual state. At the end, the model indicates that the decentralization of law enforcement activities tends to lower the overall welfare of domestic residents.

In sum, any policy discussion and research on unauthorized immigration should be based on the most accurate information available. The currently available estimates have been widely used in research to examine the drivers of unauthorized immigration and to study its impact on the domestic economy. The estimates are not completely flawless, but they are very reasonable considering the difficulties of measuring unauthorized immigration.

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